Optical recording technology that enables consumers and others to record laser-written labels on specially coated recordable CD and DVD media has enjoyed notable commercial success. In light-activated thermal label-recording technology, a surface of the medium is coated with a writable layer of a material that changes appearance when it absorbs laser light of a predetermined wavelength.
Some coating compositions for optical recording have color-forming components that include leuco-dye of a fluoran type and phenol developer or activator, both of low solubility, dispersed as separate phases in a radiation-curable polymer matrix. Typically, such formulations require imaging radiation energy density greater than 0.5-1.0 Joule/cm2.
Other coating compositions for optical recording include two separate phases: (1) a polymer matrix with acidic developer (such as a phenol) dissolved in the matrix and (2) a fluoran type of leuco-dye which is not soluble or has low solubility in the first phase at ambient temperature. With such compositions, color is developed through heating of the coating with laser radiation. In order to tune (sensitize) the coating to the laser emission wavelength used and to optimize heating efficiency, such compositions may also include an “antenna dye” with a peak absorption wavelength Lambda(max) matching the laser wavelength.
Some compositions utilize a color-forming reaction between a fluoran leuco-dye and an acidic developer such as a phenol. The reaction between the fluoran leuco-dye and a phenol developer is reversible and, hence, color formation is reversible. Certain factors, such as leuco-dye crystallization from the imaged coating or increased mobility of color-forming species in the coating matrix caused by contamination (e.g., human skin oil) may cause discoloration of the image in some of these coating compositions.
Other coating compositions have a color layer and a layer of light-scattering pigment that becomes at least translucent when heated to a predetermined temperature. In order to produce a color mark, a two-layered structure is required, having a light-scattering layer and a colored undercoat. Thus, there is a need for further improvement in coating compositions and methods for marking of media.